It is known that 2-halogen-5-methylpyridines are useful as starting materials or intermediates in the synthesis of herbicides of the pyridyloxyphenoxy type. Examples of 4-(5-halomethyl-2-pyridyloxy)phenoxy compounds useful as herbicides are disclosed in European Published Patent Application No. 483, United Kingdom Pat. Specifications Nos. 1,599,121 and 1,599,126 and U.S. Pat. Nos. 4,184,041 and 4,317,913. For example, butyl 2-[4-(5-trifluoromethyl-2-pyridyloxy)phenoxy]propionate which is also known as fluazifop-butyl is an effective grass herbicide which can be used in fields where broad-leaved crops such as cotton and soybeans are cultivated. Important starting materials for such pyridyloxyphenoxy compounds are the 2-halo-5-trichloromethylpyridines such as 2-chloro-5-trichloromethylpyridine described in U.S. Pat. No. 4,317,913. Such 2-halo-5-trichloromethylpyridines, in turn, may be prepared by chlorinating, under ultraviolet light irradiation, a 2-halo-5-methylpyridine as described in U.S. Pat. No. 4,152,328.
One object of the present invention is a new process for the synthesis of 2-halo-5-methylpyridines which are known to be useful in the synthesis of herbicides. Such a novel process would be efficient in that a single starting material (II) may be reacted via at least two alternative steps to produce at least two different intermediates which all may be taken on to produce a single compound. With such a process, one can plan on using a single starting material and producing a single final product while retaining processing flexibility by the ability to alter reagents and reaction conditions since one can carry the process through different intermediates of the formula (III) during the process of the invention.
A further object of the present invention is a process for preparing 2-halo-5-methylpyridines which includes cyclization of a 4-methylpentane derivative with an amine whereby the direct product of the cyclization has the correct oxidation state for the desired aromatic pyridine.